The Integumentary System 79 



breaking. In birds, there are but few skin glands, but the preen gland 

 located at the base of the tail provides oil for the feathers. 



The characteristic milk-producing glands of the mammal, the 

 mammary glands, become functional only after parturition (birth of 

 young). The mucous and poison glands of the amphibians are of use 

 to them for retaining moisture and for protection. There are many 

 other specialized glands found throughout all groups of vertebrates. 



9. Reproduction. Although the integumentary system does not 

 play an essential role in reproduction, it does have certain supple- 

 mentary functions. Among many animals, the color of integumentary 

 derivatives is of importance in sexual recognition. This is more con- 

 spicuous among birds and fash than among mammals. Among the 

 members of this latter group, odors produced by special glands are 

 of more importance. 



THE MICROSCOPIC STRUCTURE OF THE SKIN 



Two distinct parts form the skin. The outermost of these, the 

 epidermis, is a thin layer composed of stratified epithelial cells. The 

 inner part or dermis is much thicker and quite fibrous in structure. It 

 is within this layer that the blood vessels, nerve endings, and glands 

 are located. 



The Epidermis.— Among the higher vertebrates, the cells form- 

 ing the epidermis are arranged in layers. Those of the deeper layers 

 are cuboidal in shape, while the outermost cells are flattened. The 

 nnermost layer, the stratum germinativum or Malpighian layer, con- 

 sists of living cells which are continuously dividing to replace the outer 

 cells that slough ofif. In the thick skin forming the palms of the hands 

 and the soles of the feet, the other layers can be easily noted. These 

 cannot be seen so easily in the thinner skin of the greater part of the 

 body. As the cells of the stratum germinativum layer divide, they 

 push toward the outer surface of the skin. This outward movement 

 is accompanied by certain characteristic chemical and physical changes. 

 Just above this layer is the stratum granulosum. This name is de- 

 rived from the fact that certain granules are formed which give the 

 cells a granular appearance. Above this is another transition layer, 

 the stratum lucidum. Its name is due to the fact that the cells form- 

 ing it are somewhat transparent as the result of chemical changes that oc- 

 curred in the granular material. The outermost layer, the corneum, ,s 

 formed of dead scalelike cells without nuclei and is composed chiefly ot 



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